1. Field of the Invention
This invention relates to solid-state electrochemical cells such as solid-state batteries and solid-state analogue memory cells.
2. Description of the Prior Art
At the present, electrochemical cells such as batteries and the like that are widely used employ a liquid electrolyte. Such conventional electrochemical cells have the disadvantage of giving rise to leakage of the liquid electrolyte. To eliminate this disadvantage, solid-state electrochemical cells such as batteries and analogue memory cells that make use of a solid electrolyte have been developed. Because there is no danger of leakage of the electrolyte from a solid-state electrochemical cell, the reliability of such an electrochemical cell is greater than that of a conventional electrochemical cell that makes use of a liquid electrolyte. Also, with the use of a solid electrolyte, it is possible to produce thin electrochemical cells that are small.
Among such small, thin electrochemical cells, for example, solid-state batteries are useful as the electrical power source for backup of semiconductor memory elements. To ensure that information acquired will not be lost when the semiconductor memory element is detached from the printed circuit board, it is preferred that such a supplementary electrical power source (i.e., solid-state battery) and the semiconductor memory element be integrated in one piece and provided in the same package. A solid-state battery that is provided together with semiconductor elements in the same package is exposed to high temperatures when the package is sealed, when the semiconductor elements are soldered to the printed circuit board, and when the semiconductor elements are operated at high temperatures. Therefore, the solid-state battery must be stable at high temperatures.
As an example of a solid-state electrochemical cell, there are, for example, rechargeable batteries that make use of a copper-ion conductive solid electrolyte of the RbCl-CuCl-CuI system, and a positive-electrode active material of transition metal disulfides with a laminar structure such as TiS.sub.2 and NbS.sub.2, or copper Chevrel-phase compounds of the formula Cu.sub.x Mo.sub.6 S.sub.8. As another solid-state battery that makes use of a copper-ion conductive solid electrolyte of the RbCl-CuCl-CuI system, there are solid-state lithium batteries in which metal lithium is used as the negative-electrode active material and various oxides or sulfides are used as the positive-electrode active material.
However, the above-mentioned solid electrolyte of the RbCl-CuCl-CuI system is readily decomposed by the water and oxygen in the air. Also, sulfides that can be used as the electrode active material are readily oxidized, particularly at high temperatures For that reason, solid-state electrochemical cells such as are described above must be made in a sufficiently dry atmosphere from which oxygen has been removed. Moreover, to ensure that stability at high temperatures will be maintained, it is necessary to prevent air from the outside from entering the cell. However, the seal made of resin in conventional solid-state electrochemical cells is imperfect. Therefore, it is necessary for the same kind of seal to be made as for electrochemical cells that make use of a liquid electrolyte, or else it is necessary to provide a hermetic seal. In solid-state lithium batteries as well, it is necessary to provide protection from the air because the metal lithium is readily oxidized.
There are methods that make use of a solid electrolyte of silver compounds that are stable against the water and oxygen in the air, used to overcome these problems. For example, there are primary batteries such as iodine/silver batteries and iodine complex/silver batteries, which make use of a silver-ion conductive solid electrolyte of the formula aAgXbAg.sub.2 O-cM.sub.k O.sub.l (where X=I, Br, or Cl, and M=W, Mo, Si, V, Cr, P, or B) or pAgX-qAgM.sub.m O.sub.n (where X=I, Br, or Cl, and M=W, Mo, Si, V, Cr, P, or B). However, these kinds of batteries have the disadvantage of emitting iodine at high temperatures. An appropriate electrode active material for secondary batteries that make use of a solid electrolyte such as described above has not yet been found.
When the above-mentioned solid-state batteries are used together with a semiconductor element, the following problems can arise. With a copper-ion conductive solid electrolyte of the RbCl-CuCl-CuI system, iodine is emitted from the electrolyte at high temperatures or as the effect of the permeation of water through the packaging material. The iodine emitted stains the resin that is used in the packing of the semiconductor element, which can cause a degradation in the insulation properties of the resin and which can degrade the characteristics of the semiconductor element in this package. The abovementioned solid-state metal lithium batteries contain, as the negative electrode, metal lithium, Wood's metal, Li-Pb alloy, or the like that have a low melting point. Therefore, the negative electrode may melt at high temperatures. The melted metal may diffuse in the silicon that constitutes part of the semiconductor element, degrading the characteristics of the semiconductor element. With solid-state batteries that make use of silver-ion conductive solid electrolytes, iodine gas is emitted at high temperatures from the iodine or iodine complex used as the positive-electrode material.
The solid-state analogue memory cells that have been developed include a silver-ion electron conductor that makes use of a silver-ion conductive solid electrolyte and Ag.sub.2 Se-Ag.sub.3 PO.sub.4 electrodes. In these cells, there is a linear relationship between the amount of silver contained in the electrode and the chemical potential, and the relationship between the electricity fed to the cell and the electrode potential is also linear. However, in this system, the electricity and the electrode potential have a linear relationship only in the narrow range of electrode potentials of 0 to 100 mV; at an electrode potential of more than 100 mV, the electricity cannot be detected and stored. Moreover, there is the problem that the silver selenide used in the electrode is readily oxidized at high temperatures.